Method and Apparatus for Phase Change Enhancement

ABSTRACT

A method of operating an evaporator is described. In evaporator feed water, a Taylor bubble is developed which has an outer surface including a thin film in contact with an inner surface of an outer wall of an evaporator tube. The Taylor bubble is heated as it rises within the evaporator tube so that liquid in the thin film transitions into vapor within the bubble.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation Application of U.S. patentapplication Ser. No. 11/073,935, filed Mar. 7, 2005, and entitled“Method and Apparatus for Phase Change Enhancement, now U.S. PublicationNo. US-2005-0183832, published Aug. 25, 2005 (E27), which is aContinuation-In-Part Application of U.S. patent application Ser. No.10/636,303, filed Aug. 7, 2003 and entitled “Method and Apparatus forPhase Change Enhancement and published Apr. 22, 2004 as U.S. PublicationNo. US-2004-0074757 (D82), now abandoned, which claims priority fromU.S. Provisional Patent Application Ser. No. 60/401,813, filed Aug. 7,2002, and entitled “Method and Apparatus for Boiling Enhancement in aRising Film Evaporator” (C49), and from U.S. Provisional PatentApplication Ser. No. 60/425,820, filed Nov. 13, 2002, and entitled“Pressurized Vapor Cycle Liquid Distillation” (C48), all of which arehereby incorporated herein by reference in their entireties.

FIELD OF THE INVENTION

The present invention pertains to improvements for the conversion ofliquid to vapor, such as the phase change that takes place in anevaporator.

BACKGROUND ART

The conversion of liquid to vapor is a fundamental step in manyprocesses. For a variety of reasons, such as time and energylimitations, it may be advantageous to make this phase change moreefficient. For example, one method of achieving more efficient phasechange is though the use of thin film evaporation.

Rising film evaporation is one form of thin film boiling. When waterboils at a high enough rate in a vertical tube, the central area of thetube fills with steam. As shown in FIG. 8, under the right conditions,the upward movement of the steam will rapidly draw a thin film of waterup the inside of the tube forming a natural thin film on the inner wallof the tube. As the feed mixture moves up the inside of the tube, morevapor is formed resulting in a higher central core velocity that forcesthe remaining liquid to the tube wall. Higher vapor velocities, in turn,result in thinner and more rapidly moving liquid film. But very accuratefeed water level control (e.g., .+−.0.25″) is needed to achieve thismode of operation. Such accurate level control in a portable system maybe difficult to achieve because the system would have to be leveled towithin 1.degree. of tilt.

Thin film evaporation is typically achieved using apparatus thatincludes devices with very small openings or very small sprayingarrangements. This apparatus can easily clog, particularly when thesource liquid contains contaminants. The apparatus may also be sensitiveto movement and positioning of the apparatus. What is needed is aninvention that allows for an increase in the phase change efficiencysimilar to the efficiencies obtainable from thin film evaporation,without the limitations and sensitivities typically experienced withthin film evaporation.

For example, vapor compression distillation has proved useful forpurifying liquids, e.g., turning salt water into potable water. Suchdevices frequently employ an evaporator chamber comprising a set ofvertically oriented tubes, which tubes are heated on their exteriors.The heated tubes create vapor from a liquid that is input to the tubesthrough openings in the bottom of the tubes. The vapor that emerges fromeach tube is compressed and heat from the vapor is then transferred tothe liquid in the tubes by passing the compressed vapor over the outsideof the tubes. The vapor condenses as it transfers its heat and theresultant distillate is drawn off. A vapor compression distillationdevice is disclosed in The Naval Sea Systems Command (Sea-03Z43), NavalShips' Technical Manual, Chapter 531, Desalination Volume 2, VaporCompression Distilling Plants, # S9086-SC-STM-020/CH-531V2R2, 1 Sep.1999, which is incorporated herein by reference in its entirety. Theefficiency of a rising film evaporator can be characterized by the ratioof distillate output per unit time to the power input to the evaporatorper unit time.

In this specification and in any appended claims, unless contextrequires otherwise, the term “phase change chamber” will mean anystructure with at least one inlet end for introducing liquid and atleast one outlet end for allowing vapor to exit. The chamber is intendedto be heated externally and to allow a liquid-to-vapor phase change tooccur within. Such chambers include, without limitation, evaporatortubes, that may be cylindrical, and the parallel core layers describedabove. Other geometries as are known for such chambers to those skilledin the art are intended to be within the scope of the invention asdescribed in the claims.

SUMMARY OF THE INVENTION

In accordance with embodiments of the present invention, an improvementis provided for devices that convert liquid to vapor, such asevaporators. In evaporator feed water, a Taylor bubble is developedwhich has an outer surface including a thin film in contact with aninner surface of an outer wall of an evaporator tube. The Taylor bubbleis heated as it rises within the evaporator tube so that liquid in thethin film transitions into vapor within the bubble.

In a further embodiment, the evaporator tube includes a internal centerrod to form an annular cylinder space within the evaporator tube so thatan annular Taylor bubble is developed. The evaporator tube may furtherinclude an internal spiral wire adapted to maintain the center rod inposition within the evaporator tube. The thin film in contact with theinner surface of the of the outer wall of the evaporator tube istypically much thinner for an annular Taylor bubble than for acylindrical Taylor bubble.

In a further embodiment, heating the Taylor bubble includes heating theoutside of the evaporator tube, for example, using compressed vapor fromwithin the evaporator. The compressed vapor may also be cooled to formcondensed distillate output of the evaporator.

Embodiments of the present invention also include an evaporator adaptedto use any of the foregoing methods.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a tube-type evaporator.

FIG. 2 shows the rate of distillate output for an evaporator as afunction of pressure for several liquid boiling modes.

FIG. 3 illustrates an evaporator tube incorporating a rod as packing toenhance boiling of a liquid in a rising film evaporator.

FIG. 4 illustrates an evaporator tube incorporating a brush as packingto enhance boiling of a liquid in a rising film evaporator.

FIG. 5 shows a comparison of the rate of distillate output as a functionof pressure for an evaporator for pool boiling and for a tube with rodand wire mesh packing.

FIG. 6 shows a comparison of the rate of distillate output as a functionof pressure for an evaporator for pool boiling and for a tube with fulland half packing.

FIG. 7 shows a comparison of the rate of distillate output as a functionof pressure for tubes packed with rods of varying diameters.

FIG. 8 illustrates the heat flow principles in embodiments that use slugflow of the feed water based on Taylor bubbles.

FIG. 9 shows an evaporator tube having an internal spiral wire tomaintain the position of a center rod.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Various embodiments of the present invention are directed to techniquesfor enhancing the efficiency of phase change for liquids, such as in anevaporator. As used in this application, the term “boiling” will beunderstood to include a phase change between liquid and vapor where nobubbles are formed, as well as a phase change where bubbles are formed.

FIG. 1 shows an evaporator 10 for distilling a liquid according to anembodiment of the present invention. The evaporator includes a set 20 ofcylindrical evaporator tubes 21 that are substantially verticallyoriented. Liquid is introduced to each tube through an inlet at thebottom of each tube. Each tube includes a heated central region 25 forboiling the liquid and producing vapor. Each tube has a vent openingthat allows vapor to escape from the tube into an evaporation chamber30. Liquid that has not undergone phase change also escapes through thevent opening into the chamber where the liquid may be recirculated tothe tube inlets or removed from the device.

The central region of the evaporator tubes may be heated by any ofseveral means. One means is by compressed vapor, e.g. steam, in contactwith the exterior of each tube. A compressor 35, which may be a liquidring compressor, compresses vapor drawn from the evaporation chamber 30,raising the vapor's pressure and temperature. The compressed vapor ischanneled to the exterior of the evaporator tubes in the central region.The compressed vapor condenses around the evaporator tubes therebyheating the liquid in the tubes to boiling. The distillate from thecondensed vapor is then drained off.

Typically, an evaporator may operate in either of two modes: poolboiling mode or thin film mode. In thin film boiling, a thin film ofliquid is created on the inner wall of the tubes facilitating heattransfer from the tube wall to the free surface of the liquid. Theefficiency of phase change typically increases for thin film mode ascompared to pool boiling mode. FIG. 2 shows the difference in the rateof distillate production as a function of condenser pressure for poolboiling and thin film boiling under similar conditions for arepresentative evaporator. The bottom curve 70 corresponds to poolboiling while the middle curve 75 corresponds to thin film boiling. Aswill be noted from these two curves, thin film boiling mode offerssignificantly higher efficiency than pool boiling mode. Thin filmboiling is more difficult to maintain than pool boiling, however. Thinfilm evaporation is typically achieved using apparatus that includesvery small openings. This apparatus can easily clog, particularly whenthe source liquid contains contaminants. Additionally, in thin film modethe water level is typically held just marginally above the tops of thetubes in a vertical tube-type evaporator. For reasons such as this, theapparatus may also be sensitive to movement and positioning of theapparatus.

Improved efficiency of a phase change operation is achieved inaccordance with embodiments of the present invention by providingpacking within the evaporator tubes 21. The introduction of such packingmay allow the evaporator to take on some of the characteristics of thinfilm mode, due to the interaction between the liquid, the packing andthe heating tube. The packing may be any material shaped such that thematerial preferentially fills the volume of a tube near the tube'slongitudinal axis versus the volume near the tube's interior wall. Suchpacking material serves to concentrate the vapor near the walls of thetube for efficient heat exchange. For example, in an embodiment of thepresent invention shown in FIG. 3, the packing may comprise a rod 40 ora plurality of rods inserted into an evaporator tube 21. Each rod 40 maybe of any cross-sectional shape including a cylindrical or rectangularshape. The cross-sectional area of each packing rod 40 may be any areathat will fit within the cross-section of the tube. The cross-sectionalarea of each rod may vary along the rod's length. A given rod may extendthe length of a given evaporator tube or any subset thereof.

Each rod may be positioned anywhere within the tube includingpreferentially in the upper portion of the tube. In a specificembodiment, each rod is approximately half the length of the associatedtube and is positioned approximately in the top half of the tube. Agiven rod may be made of any material including, for example, a metal,nylon, Teflon or plastic and in certain embodiments may be hydrophobic.The top curve 80 in FIG. 2 shows the increase in boiling efficiency forthin film boiling for a representative evaporator where the evaporatortubes include packing material in approximately the top half of thetubes. With such packing, the phase change efficiency is also,advantageously, much less sensitive to changes in the liquid level abovethe tubes, the orientation of the tubes with respect to the vertical,the feed pressure for the tubes and other operating parameters for theevaporator.

In a specific embodiment of the present invention, as shown in FIG. 4,the packing is in the form of a rod 50 with bristles 52 emanatingtherefrom, forming a brush 55. The length of the bristles is determinedso that a subset of the bristles contacts the inner surface of the tube,when the brush 55 is inserted into the tube. As used in this descriptionand in any appended claims, the word “subset” shall include both propersubsets and a subset that includes every member of the set in question.The brush inserted in any given tube may extend the length of the tubeor any portion thereof. Each brush may be positioned anywhere within thetube including at the upper end of the tube. In a specific embodiment,each brush is approximately half the length of the associated tube andis positioned approximately in the top half of the tube. In anotherembodiment of the invention, the brush is positioned and the length ofthe bristles is such that none of the bristles contact the evaporatortube wall. In other embodiments of the invention, the packing may be amesh or other loose packed material.

As an example, an evaporator was built with 10 tubes, with each tube1.25 inches in diameter and 18 inches in length. The distillation rateas a function of condenser pressure was measured with a variety ofpacking in the evaporator tubes. For example, FIG. 5 shows thedistillation rate for no packing in the tubes (i.e., pool boiling mode)for a mesh packing which completely filled the tube and for packingconsisting of rods. The graph clearly shows that the rod packingsignificantly enhanced the output rate of the evaporator as compared topool boiling while the mesh provided a much less significant improvementin output rate as compared to pool boiling. FIG. 6 compares the outputfor evaporator tubes with a rod inserted for its full length and with arod inserted for half of its length. As can be readily seen, the outputrate appears to be insensitive to the length of the rods in these twocases. Finally, FIG. 7 shows the results from inserting rods with 0.875inch, 1.00 inch and 1.125 inch diameters respectively into the upperhalf of the evaporator tubes. As can be seen, the output is maximizedfor the intermediate diameter rod (1.00 inch). This phenomenon may bedue to the intermediate diameter rod allowing the flow rate of steampast the rod to be increased as compared to the smallest rod (0.875inch), while avoiding the restricted flow past the rod that the largestrod (1.125 inch) may provide.

In other embodiments, the evaporator or condenser may include formatsother than tubes, such as the flat evaporator/condenser disclosed inU.S. provisional patent application Ser. No. 60/425,820, filed Nov. 13,2002, entitled “Pressurized Vapor Cycle Liquid Distillation,”incorporated herein by reference in its entirety. Such flatevaporator/condensers typically contain multiple parallel core layers,with rib sections creating channels for directing steam and condensedliquid flow. In this embodiment, the improvement comprises insertingpacking material inside a given evaporator layer to improve the net rateof phase change. The packing may be any material suitable for use withthe given liquid under the conditions typically found in an evaporatorand may be placed along the entire length of the evaporator layer or anyportion thereof. In this embodiment, the packing may be shaped such thatthe material preferentially fills the center of the evaporator layer andmay be of any thickness less than the thickness of the evaporator layer.The packing may be any solid or hollow shape or may comprise a rod witha plurality of bristles emanating from the rod. In a specificembodiment, the length of the bristles is set so that at least a subsetof the bristles contact both the upper and lower surface of theevaporator layer. In another embodiment, the packing material may be aloosely packed material, such as wire mesh, inserted into the evaporatorlayer.

In yet another embodiment of the invention, rather than insertingpacking material into an evaporator tube or a flat layer of anevaporator/condenser, the evaporator may be fabricated to achievesimilar results with respect to increased efficiency. For example, in anevaporator containing substantially vertical tubes, the tubes may beformed with a permanent cylindrical section, similar to a rod, placed inthe center of the tube. Additionally, for example, a flatevaporator/condenser may be formed with plates that are placed atappropriately spaced intervals to achieve a similar result to the use ofpacking materials.

An embodiment based on a tube with a center cylindrical rod (as shown inFIG. 3) may use a process based on what is referred to as slug flowbased on formation of large bullet shaped vapor (Taylor) bubbles in theincoming feed water that fill most of the inner tube cross section.Because of the center rod inside the tube, these vapor pockets willactually be in the form of annular Taylor bubbles.

As shown in FIG. 8, as an individual Taylor bubble 80 rises verticallywithin the tube, a thin film 81 forms the outer boundary of the bubbleand flows in contact with the inner surface 82 of the outer tube wall83. And in the embodiment shown in FIG. 8, the outer surface 84 of theouter tube wall 83 is heated by compressed vapor at a temperature abovethe transition temperature of the inner thin film 81. The loss of heatenergy from the compressed vapor external to the tube causes it tocondense forming a film of liquid distillate 85 which falls along theouter surface 84 of the outer tube wall 83. Thus, there is a power fluxfrom the outer surface 84 of the outer tube wall 83 (in contact with thecondensing compressed vapor) across the outer tube wall 83 to its innersurface 82 (in contact with the inner thin film 81 of the Taylor bubble80). As the Taylor bubble 80 rises within the tube, the thin film 81 isheated by the inner surface 82 of the outer tube wall 82 it contacts.This causes the fluid that forms the thin film 81 to transition to vaporwhich adds to the vapor that forms the Taylor bubble 80. Although thethin film 81 of the Taylor bubble 80 is relatively thin, the upward flowvelocity of the bubble 80 is lower than what is required for the risingfilm form of distillation.

Slug flow based on Taylor bubbles can be the principle used in anevaporator system such as the one shown in FIG. 1. For example, theremay be 85 cylindrical evaporator tubes 21 in the set 20 of tubes. In onespecific embodiment, the entire set 20 carries 15 g/sec of feed water,which is a relatively low flow rate of the type that promotes laminarflow along the various internal surfaces of the tubes. Each tube 21contains an internal center rod 40 as is shown in FIG. 3 so that theinterior of each tube forms an annular cylinder. In such a structuralgeometry, the slug flow mode will form annular Taylor bubbles.Experimental observations indicate that the thin film 81 is dramaticallythinner for an annular Taylor bubble than for other shapes. Such verythin films exhibit significantly better heat transfer characteristicsand vapor formation qualities than thicker films, and it is therefore animportant aspect of embodiments of the present invention that the Taylorbubble be annular shaped in order to achieve the necessary thinness ofthe fluid film around the bubble.

In some embodiments, there may further be within each tube 21 a loosespiral wrap wire 90 as shown in FIG. 9. This may help to keep the centerrod 40 centered within the tube 21 while not interfering with the slugmode and Taylor bubble process. In other embodiments, the center rod 40may include a series of external vanes that help keep the rod centeredwithin the tube 21, but such an arrangement might interfere with Taylorbubble process, for example by dividing the annular cylinder so as toprevent a single annular bubble from forming across a given crosssection of the tube 21.

And as before, the central region 25 may be heated by compressed vapordrawn from the evaporator chamber 30 which both raises the temperatureof the feed water flowing through the inside of the tubes 21 in slugflow mode with Taylor bubbles, and also lowers the temperature of thecompressed vapor which condenses into the desired distillate, whichforms the output of the evaporator 10.

In specific embodiments of the present invention, the incoming feedwater has high overall fluid enthalpy and is near the transition pointfrom liquid to vapor. Thus, there is only a small temperature gradientalong the length of the tube. And there may also be only a smalltemperature gradient across the condensing film, the thickness of thetube from the outer surface to the inner wall, and across theevaporating film to the free surface of the film around the bubble. As aresult, such embodiments are highly efficient so that minimum power isrequired to produce the desired distillate.

Although various exemplary embodiments of the invention have beendisclosed, it should be apparent to those skilled in the art thatvarious changes and modifications can be made which will achieve some ofthe advantages of the invention without departing from the true scope ofthe invention.

1. An evaporator for distilling a liquid comprising: at least oneevaporator tube wherein the at least one evaporator tube having a liquidinlet, a vent, a longitudinal axis and an interior wall; and packingmaterial within at least one portion of the length of the at least oneevaporator tube, wherein the packing material located along the tube'slongitudinal axis.
 2. The evaporator of claim 1, further comprisingwherein the packing material is at least one rod.
 3. The evaporator ofclaim 2, further comprising wherein the at least one rod is cylindrical.4. The evaporator of claim 2, further comprising wherein the at leastone rod is rectangular.
 5. The evaporator of claim 2, further comprisingwherein the cross-sectional area of the rod varies along the rod'slength
 6. The evaporator of claim 2, further comprising wherein the rodextends along the length of the evaporator tube.
 7. The evaporator ofclaim 2, further comprising wherein the rod is approximately half thelength of the evaporator tube.
 8. The evaporator of claim 7, furthercomprising wherein the rod is positioned approximately in the top halfof the evaporator tube.
 9. The evaporator of claim 2, wherein the rodfurther comprising bristles emanating from the rod to form a brush. 10.An evaporator for distilling a liquid comprising: at least oneevaporator core layer having an upper and lower surface, wherein the atleast one evaporator core layer having a plurality of rib sections, theplurality of rib sections forming channels for directing flow; andpacking material within at least one portion of the evaporator corelayer.
 11. The evaporator of claim 10, further comprising wherein thepacking material is at least one rod.
 12. The evaporator of claim 11,further comprising wherein the at least one rod is cylindrical.
 13. Theevaporator of claim 11, wherein the rod further comprising bristlesemanating from the rod to form a brush.
 14. The evaporator of claim 13,the bristles further comprising a length wherein at least one bristlecontacts both the upper and lower surface of the core evaporator layer.15. An evaporator for distilling a liquid comprising: at least oneevaporator tube wherein the at least one evaporator tube having a liquidinlet, a vent, a longitudinal axis and an interior wall; and at leastone cylindrical rod within at least one portion of the length of the atleast one evaporator tube, wherein the at least one cylindrical rodlocated along the tube's longitudinal axis.
 16. The evaporator of claim15, further comprising wherein the cross-sectional area of the rodvaries along the rod's length
 17. The evaporator of claim 15, whereinthe rod further comprising bristles emanating from the rod to form abrush.
 18. The evaporator of claim 15, further comprising wherein therod extends along the length of the evaporator tube.
 19. The evaporatorof claim 15, further comprising wherein the rod is approximately halfthe length of the evaporator tube.
 20. The evaporator of claim 15,further comprising wherein the rod is positioned approximately in thetop half of the evaporator tube.